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Pros

Pro

Widely used

De facto standard for log aggregation, monitoring, analysis and reporting.

Pro

Easy to use, powerful search

It's human readable, intuitive, fast and with auto-complete to boot. And if you need more advanced functionality out of your queries, 120+ page search manual will give you an insight in how much is actually possible.

Pro

Free version

With restrictions on log size and devices a free version is available.

Pro

Alerts

Splunk can turn searches into alerts.

Pro

Cross-platform

OS X, Linux & Windows are supported. You can also access Splunk from iOS and Android devices.